1. Field of the Invention
This invention relates to a device for monitoring the continuity of a wrist-strap connection, and particularly a device of such type which is useful for monitoring single-conductor wrist straps.
2. Description of the Related Art
A famous gadget used by the electronics industry worldwide is known as the "wrist-strap". Essentialy, a wrist-strap is a piece of wire, or a plurality of wires, used to preclude the build-up of static electricity on personnel by "grounding" the human body, i.e., by attaching the human body to earth via the wrist.
In the past, single-wire wrist straps were used extensively. However, in the recent years such straps were quickly phased-out and replaced by two-wire straps, as it was found out that no means existed to monitor the continuity of the wrist-strap connection; i.e., to monitor the validity of the connection between the human body and the earth, with a single-wire system.
As it was found-out, it was necessary to use at least two wires in order to monitor the continuity of such connection. This is typically achieved by means of circulating a current from the ground point to the human body, via the first wire, and then back from the body to the ground point, via the second wire, thereby establishing a closed current loop. Means was then inserted into the loop in order to detect the continuous presence of the current. Any discontinuity in the current would then mean a broken connection. This idea can be found in two key patents: U.S. Pat. No. 4,605,984 issued to Robert Fiedler and assigned to Beckman Industries, and the group of U.S. Pat. Nos. issued to Albert Breidegam (4,639,825, 4,745,519, 4,813,459) and assigned to Semtronics Corp.
The Fielder patent shows an AC source being used to circulate a current through the closed loop formed by two wires, where an LCD display senses the presence of the AC signal as an indication of continuity. The Breidegam patents show essentially the same idea being used in conjunction with a DC circuit. The Breidegam patents further show the possibility of testing the resistance of the skin of the wearer to ensure that the wrist-strap is in firm contact with the body of the wearer.
This prior-art idea, while useful for continuous monitoring of the wrist-strap connection, has several deficiencies: first, since the continuity of the loop requires that the current flows to the strap, then to the skin of the user, and then back to the strap, this makes the design of the wrist-strap assembly complicated, and hence more expensive. Secondly, since the user must maintain good electrical contact with two conductors inside the strap, instead of one, the wrist-strap must be very tightly :secured around the wrist of the wearer all the time, which is an annoying requirement for most users.
Another well known device, manufactured by Pilgrim Corp., is based on the idea of detecting the capacitance of the human body in order to ensure continuous presence of the body. This idea is not very popular due to the fact that capacitance measurement is very sensitive to noise, and hence unreliable.
It is the objective of the present invention to overcome the deficiencies of the prior-art by providing a successful device for wrist-strap monitoring which is useful for single-conductor wrist-straps. Such device reduces the cost and the complexity associated with two-conductor straps. It is another objective of the present invention to provide a wrist-Strap monitoring system which is very flexible and convenient for the user, yet efficient and reliable.
Other objectives and features of the invention will become fully apparent from the ensuing disclosure and appended claims.